Electric heat control system

ABSTRACT

The system of the invention includes a bridge circuit adapted to sense changes in the desired and set temperature of apparatus at a remote location. The system includes means for sensing unbalance in the bridge due to a change in the temperature of the apparatus at the remote location, the circuit including means for responding to the bridge unbalance and firing or operating a control circuit and a heating circuit for heating the apparatus.

United States Patent Kerchner [45] Mar. 14, 1972 [541 ELECTRIC HEATCONTROL SYSTEM 3,149,224 9/1964 Home et al.... ..219/501 X 3,440,3974/1969 Vesper et a]... ..2l9l501 X [72] Invent Bath 3,407,286 10/1968Roby ..219/501 [73] Assignee: Kalglo Electronics Co., Inc., Lehigh Val-3,514,628 5/1970 Pinckaers ..219/494 X ley, Pa. Primary Examiner-JohnZazworsky 22 Fld. A .151970 E211 A l N 2:259 Attorney-Robert A. Green[57] ABSTRACT I [52] [1.8. Cl 307/310, 219/499, 219/501, h system of theinvention includes a bridge drcuit adapted 307/252 B to sense changes inthe desired and set temperature of ap- [51] lIPt. Cl ..H03k 17/00, 05b1/02 paratus at a remote location The System includes means for [58]held of Search 5 sensing unbalance in the bridge due to a change in thetemperature of the apparatus at the remote location, the circuitincluding means for responding to the bridge unbalance and- [56]References Cited firing or operating a control circuit and a heatingcircuit for UNITED STATES PATENTS heating apparatus- 3,341,769 9/1967Grant ..219l501 10 Claims, 1 Drawing Figure BACKGROUND OF THE INVENTIONIn the past, heating in an agricultural environment, for example, formaintaining temperature in chicken coops, pig pens, and the like, hasbeen accomplished with equipment which used gas as a fuel rather thanelectricity. This has been done for economic reasons. Electric heat isdesirable for a number of well known reasons; however, electric heatingapparatus presently available is generally not satisfactory foragricultural or farm use because it is too sophisticated and tooexpensive to install and operate.

Various attempts have been made to achieve an economical method ofheating electrically in an agricultural environment, the mostsignificant being in the use of infrared heaters. However, with the useof this type of heating unit, since heat is applied the instant the heatlamps are energized and removed the instant they are deenergized asimple on-off thermostat cannot be used, or the animals will senseuncomfortable hot and cold periods. Thus, an electronic system whichincludes, among other things, means for modulating the heating voltageis required, and this is not known in the prior art.

The present invention provides a relatively inexpensive electronic heatcontrol circuit which modulates the applied voltage, is simple tooperate, and provides no failures, long term reliability, and can bepackaged so that it is resistant to environmental damage.

SUMMARY OF THE INVENTION Briefly, the electronic system of the inventioncomprises a resistance bridge which senses the temperature state at aremote location and includes associated means for applying appropriateportions of an input voltage pulse to the heating circuit.

DESCRIPTION OF THE DRAWINGS The single FIGURE is a schematicrepresentation of a system embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A system 10 embodying theinvention and shown in the drawing includes primary buses 20 and 30having input terminals 40 and 50 which are connected to a conventional240 volt, 60 cycle power supply. The primary winding 60 of a transformer70 is connected between the buses 20 and 30, and a first indicator light80 is connected between the buses 90 and 100 coupled to the ends of thesecondary winding 110 of the transformer 70. The bus 90 is coupled to abus 92 through the parallel combination of resistor 120, indicator light130, and primary winding 140 of a transformer 150 having a secondarywinding 160 which is provided with terminals 170 and 180. The buses 92and I are coupled through a full wave rectifier system including diodesD1, D2, D3, and D4, as is well known in the art, to bus 190 and bus 200which are coupled to a portion of the system 10 which is called thecontrol portion.

A resistance bridge circuit 210 is connected between the buses 190 and200, and it includes a first leg which comprises resistor 220 andresistor 230 having a junction point A between them, and a second legincluding resistor 240, thermistor 250, variable resistor 260 andresistor 270. Thermistor 250 and resistor 260 have a junction point B.Resistor 260 is used to set the desired temperature of the remoteheating apparatus represented at 280, and thermistor 250 is also locatedat the site of the heating apparatus in a suitable radiation sensingenclosure. The bridge 210 also includes series-connected PNP-NPNtransistors 290 and 300 connected between points A and B, with point Bbeing connected to the base of transistor 290, the emitter of transistor290 being connected to point A, the collector of transistor 290 beingconnected to the base of transistor 300, the collector of transistor 300being connected to the emitter of transistor 290, and the emitter oftransistor 300 being connected through resistor 310 and capacitor 320 tobus 200.

The bridge 210 also includes a unijunction transistor 330, known as aUJT, the emitter of which is connected to the junction of resistor 310and capacitor 320, one base of which is connected through resistor 340to junction point A, and the second base of which is connected throughresistor 350 to the bus 200. An output terminal 360 is connected to thesecond base of unijunction transistor 330. The position of the UJTcircuit between point A and bus 200 eliminates the need for a Zenerdiode across the whole bridge, as might be required in anotherarrangement.

The output terminal 360 associated with unijunction transistor 330 isconnected to the gate of triac 370 which is connected as shown betweenthe buses I and 200.

The bus 20 is connected through the heating element 280 of the remoteheating device and a radio frequency inductor 380 to one end of a triac390, the other end of which is connected to bus 30. A capacitor 383which operates in conjunction with inductor 380 is connected across theinductor and triac 390 as shown. The terminals and of the secondarywinding 160 of transformer 150 are connected to the primary winding 400of a standard control power transformer 410, the secondary winding 420of which is connected to the gate of triac 390 and through a combinationof a resistor 430 in parallel with a series-connected resistor 433 andcapacitor 436 to one terminal of the triac 390 and to bus 30.

According to the invention, the secondary winding 1600f transformer 150can also be connected to other slave heating circuits including standardcontrol power transformers 410 and triacs 390 and other remote heatingload elements 280, as shown. If a triac such as RCA 2N5442 is used withproper heat-sinking, this circuit can control up to 8,000 watts of powerper triac. Substantially any number of remote heating circuits may becontrolled in this way, and this is desirable for large agriculturalheating requirements.

In operation of the circuit 10, transformer 150 receives a 240 voltinput and provides a reduced potential of 24 volts AC at its secondary,and this is applied across pilot light 80 which lights up and indicatesthat power is on. The diodes form a full wave rectifier and provide aseries of voltage pulses acros the bridge 210. The high resistance pathof the bridge prevents the output indicator light 130 from lighting, andinsufiicient volt age is present to energize triac 390 throughtransformer 150.

If the bridge is unbalanced because the terrnistor 250 calls for heat,as the input pulse is generated between lines and 200 across the bridge,at some time, point B goes positive and reaches a point at whichtransistors 290 and 300 conduct and charge capacitor 320. The point onthe input pulse at which the transistors begin to conduct is determinedby the potential at B, and this is determined by the unbalance of thebridge as determined by the temperature at the thermistor and thesetting of resistor 260. When the charge on capacitor 320 reaches apredetermined value, the UJT 330 fires and discharges the capacitor anda positive potential appears at point 360 and at the gate of triac 370.Triac 370 fires, the voltage across the bridge 210 drops to zero, andthe remainder of the voltage pulse at the secondary of transformer 70 isdelivered to pilot light 130. The average output and brightness of thepilot light 130 depends on the ratio of delay angle to conduction angle.Pilot lights 80 and 130 provide an inexpensive and simple-to-operatevisual reference system for indicating circuit operation. Since thepilot lights are located in the low voltage portion of the circuitinstead of across the power input and output buses as in conventionalcircuits, low cost lamps with extremely long life expectancy can beutilized.

The voltage applied across light 80 appears across transformer 150. Thisproduces a pulse across the secondary winding of transformer 150 whichfires triac 390 which conducts the remainder of the voltage cyclethrough the heater load 280. This operating cycle is repeated with eachinput pulse until the heater reaches the desired temperature and thebridge is balanced. Presumably, during subsequent input pulses, thebridge will not be unbalanced and triacs 370 and 390 will not beenergized.

Under other circumstances, heat lost and heat resupplied in each halfcycle may be balanced so that transistors 290 and 300 will be turned onlate in the input pulse and only a small burst of power will flow to theload.

It is noted that, if slave heating load circuits are provided, they,too, will receive heating power in the manner described above.

in the system shown, resistor 310 limits the emitter current oftransistor 300, and limits the minimum delay angle at which the UJT 330will fire and operate the heating circuit. This is necessary since, atvery low temperatures, which might occur at initial turn-on of theheater, thermistor 250 has very high resistance. lf resistor 260 was setto zero resistance for maximum power output, transistors 290 and 300would saturate. This would cause gating of triac 370 too early in thehalf cycle, and this would cause misfiring. Resistors 220 and 230 shouldbe relatively low in value (1) so that the shunting effect of the UJTand the phase shift networks will have minimum loading effect and (2) toprovide the current necessary to supply the energy needed to chargecapacitor 320.

Resistor 240 protects transistor 290 by providing isolation from directsupply voltage if thermistor 250 is shorted.

The circuit has a fail-safe feature in that, if thermistor 250 opens,power comes on full. This occurs because the infinite resistance of theopen thermistor indicates low temperature. This type of fail-safeoperation is desirable, instead of conventional arrangement of openthermistor-off circuit, since, if an animal bit off the control cable,the heat would not go off.

Resistor 120 is a low resistance, high power resistor to provide areliable load for triac 370 to ensure proper circuit operation in case,for example, pilot light 130 should burn out. Also, it improves thepower factor of the load of triac 370, thereby eliminating thepossibility of false turnoff of the triac.

The capacitor 436 provided in the secondary winding of transformer 410provides an initial leading inrush of current to trigger the triac 390and the slave triacs 390 more reliably at low peak voltage conductionangles.

What is claimed is:

1. An electronic heat control circuit comprising an input power source,

a load and a first gate connected across said power source,

said input power source being coupled to the primary winding of atransformer having a secondary winding,

a pilot light connected across said secondary winding,

one end of said secondary winding being connected to one end of a firstcircuit including a resistor, a pilot light, and the primary winding ofa power transformer, all connected in parallel,

said power transformer being coupled to said first gate and said load,

the other end of said secondary winding and the other end of said firstcircuit being coupled through a full wave rectifier across a resistancebridge circuit,

said bridge circuit including first and second legs, one of whichincludes fixed resistors and the other of which includes a thermistorand a variable resistor, a cross arm between said legs including activecircuit elements which respond to bridge unbalance to generate a signal,

a normally blocking second gate connected across said bridge,

said active circuit elements including means coupled to said second gatefor unblocking said gate and permitting current to flow therethroughwhereby voltage is coupled through said power transformer to said firstgate whereby said first gate is operated and unblocked to permit inputpower to be applied to said load.

2. The circuit defined in claim 1 wherein said active elements include apair of transistors coupled to a capacitor which is charged when saidtransistors turn on, and a unijunction transistor coupled to saidcapacitor and having an output coupled to said second gate for operatingthe same.

3. An electronic heat control system comprising an alternating currentvoltage source providing a cyclically varying input voltage,

a first transformer having a primary winding and a secondary windingwith said primary winding being connected across said voltage source,

a second transformer having a primary winding and a secondary windingwith the primary winding being connected in series with the secondarywinding of said first transformer and the secondary winding beingconnected to a load circuit, and

a variable resistance circuit connected across the series combination ofsaid secondary winding of said first transfonner and the primary windingof said second transformer, the resistance of said variable resistancecircuit determining whether current flows through the second transformerto said load circuit in any portion of the input signal from saidvoltage source.

4. The system defined in claim 3 wherein said variable resistancecircuit comprises a Wheatstone bridge and circuit means are coupledthereto for reducing the effective resistance of said bridge to zero ata selected point during each cycle of said input alternating current, ifsaid bridge is unbalanced, whereby operating voltage can be applied tosaid second transformer and to said load circuit.

5. The system defined in claim 3 wherein a plurality of load circuits inparallel are connected across the secondary winding of said secondtransfonner.

6. The system defined in claim 3 wherein said load circuit includes athird power transformer coupled between said second power transformerand a series-connected switch and heating load element.

7. The system defined in claim 5 wherein each said load circuit includesa third power transformer coupled between said second power transformerand a series-connected switch and heating load element.

8. The system defined in claim 3 and including a first pilot lightacross the secondary winding of said first transformer and a secondpilot light across the primary winding of said second transformer, saidfirst pilot light indicating the presence of an input signal and saidsecond pilot light indicating the state of conduction of said loadcircuit.

9. The system defined in claim 4 wherein said bridge includes acapacitor coupled thereto and chargeable due to current flow therefromwhen said bridge is unbalanced,

a first switch coupled to said capacitor and adapted to operate andgenerate a pulse when said capacitor reaches a selected charged state,

a second switch coupled to said first switch and adapted to be operatedthereby to short circuit said bridge and to thereby permit current toflow through said second power transformer.

10. An electronic heat control system comprising a source of input powerconnected to a first transformer,

a second power transformer in series with said first transformer,

a resistance bridge circuit including a first gate connected across saidfirst and second power transformers,

a load circuit including a third power transformer and a second gateconnected across said second power transformer,

said bridge circuit drawing current preferentially rather than said loadcircuit when said bridge circuit is unbalanced,

said bridge circuit including active circuit elements responsive tobridge circuit unbalance to generate a signal and to couple said signalto said first gate and thereby to operate said first gate, operation ofsaid first gate causing input power to flow through said second powertransformer and then to said third power transformer to said second gatefor operating the same and thereby permitting power from said inputsource to fiow to said load circuit.

1. An electronic heat control circuit comprising an input power source,a load and a first gate connected across said power source, said inputpower source being coupled to the primary winding of a transformerhaving a secondary winding, a pilot light connected across saidsecondary winding, one end of said secondary winding being connected toone end of a first circuit including a resistor, a pilot light, and theprimary winding of a power transformer, all connected in parallel, saidpoWer transformer being coupled to said first gate and said load, theother end of said secondary winding and the other end of said firstcircuit being coupled through a full wave rectifier across a resistancebridge circuit, said bridge circuit including first and second legs, oneof which includes fixed resistors and the other of which includes athermistor and a variable resistor, a cross arm between said legsincluding active circuit elements which respond to bridge unbalance togenerate a signal, a normally blocking second gate connected across saidbridge, said active circuit elements including means coupled to saidsecond gate for unblocking said gate and permitting current to flowtherethrough whereby voltage is coupled through said power transformerto said first gate whereby said first gate is operated and unblocked topermit input power to be applied to said load.
 2. The circuit defined inclaim 1 wherein said active elements include a pair of transistorscoupled to a capacitor which is charged when said transistors turn on,and a unijunction transistor coupled to said capacitor and having anoutput coupled to said second gate for operating the same.
 3. Anelectronic heat control system comprising an alternating current voltagesource providing a cyclically varying input voltage, a first transformerhaving a primary winding and a secondary winding with said primarywinding being connected across said voltage source, a second transformerhaving a primary winding and a secondary winding with the primarywinding being connected in series with the secondary winding of saidfirst transformer and the secondary winding being connected to a loadcircuit, and a variable resistance circuit connected across the seriescombination of said secondary winding of said first transformer and theprimary winding of said second transformer, the resistance of saidvariable resistance circuit determining whether current flows throughthe second transformer to said load circuit in any portion of the inputsignal from said voltage source.
 4. The system defined in claim 3wherein said variable resistance circuit comprises a Wheatstone bridgeand circuit means are coupled thereto for reducing the effectiveresistance of said bridge to zero at a selected point during each cycleof said input alternating current, if said bridge is unbalanced, wherebyoperating voltage can be applied to said second transformer and to saidload circuit.
 5. The system defined in claim 3 wherein a plurality ofload circuits in parallel are connected across the secondary winding ofsaid second transformer.
 6. The system defined in claim 3 wherein saidload circuit includes a third power transformer coupled between saidsecond power transformer and a series-connected switch and heating loadelement.
 7. The system defined in claim 5 wherein each said load circuitincludes a third power transformer coupled between said second powertransformer and a series-connected switch and heating load element. 8.The system defined in claim 3 and including a first pilot light acrossthe secondary winding of said first transformer and a second pilot lightacross the primary winding of said second transformer, said first pilotlight indicating the presence of an input signal and said second pilotlight indicating the state of conduction of said load circuit.
 9. Thesystem defined in claim 4 wherein said bridge includes a capacitorcoupled thereto and chargeable due to current flow therefrom when saidbridge is unbalanced, a first switch coupled to said capacitor andadapted to operate and generate a pulse when said capacitor reaches aselected charged state, a second switch coupled to said first switch andadapted to be operated thereby to short circuit said bridge and tothereby permit current to flow through said second power transformer.10. An electronic heat control system comprising a source of input powerconnected to a first tranSformer, a second power transformer in serieswith said first transformer, a resistance bridge circuit including afirst gate connected across said first and second power transformers, aload circuit including a third power transformer and a second gateconnected across said second power transformer, said bridge circuitdrawing current preferentially rather than said load circuit when saidbridge circuit is unbalanced, said bridge circuit including activecircuit elements responsive to bridge circuit unbalance to generate asignal and to couple said signal to said first gate and thereby tooperate said first gate, operation of said first gate causing inputpower to flow through said second power transformer and then to saidthird power transformer to said second gate for operating the same andthereby permitting power from said input source to flow to said loadcircuit.